Alexis Lycourghiotis

The Role of the Liquid¿Solid Interface in the Preparation of Supported Catalysts

The contribution of the Interface Science to the preparation of supported catalysts during the last two decades is presented. It is illustrated how the concepts and the methodologies of the Interface Science could be effectively used for an in‐depth understanding of the phenomena involved in the initial preparation step. This, extremely critical step, concerns the deposition of transition metal species containing the active elements, from an impregnation solution, onto the surface of common catalytic supports. Moreover, the aforementioned concepts and methodologies allow the regulation of the mode of interfacial deposition and the local structure of the deposited species and, thus, the surface characteristics and the catalytic behaviour of the resulted catalysts.

Titanium Dioxide (Anatase and Rutile): Surface Chemistry, Liquid–Solid Interface Chemistry, and Scientific Synthesis of Supported Catalysts

Solid Interface Chemistry, and Scientific Synthesis of Supported Catalysts Kyriakos Bourikas,† Christos Kordulis,‡,§ and Alexis Lycourghiotis*,‡ †School of Science and Technology, Hellenic Open University, Tsamadou 13-15, GR-26222 Patras, Greece ‡Department of Chemistry, University of Patras, GR-26500 Patras, Greece Institute of Chemical Engineering and High-Temperature Chemical Processes (FORTH/ICE-HT), P.O. Box 1414, GR-26500 Patras, Greece

Molybdenum-oxo species deposited on alumina by adsorption: I. Mechanism of the Adsorption

Abstract The mechanism of adsorption of molybdates on the surface of γ-alumina was investigated by use of adsorption equilibrium experiments, potentiometric titrations, microelectrophoresis and UV/VIS spectroscopy. From the experimental results and the theoretical analysis of the isotherms it was concluded that Mo VI deposition was done by adsorption of the Mo x O y z − ions on energetically equivalent and distinct sites of the inner Helmholtz plane (IHP) of the double layer on the surface of the γ-alumina particles suspended in the aqueous medium. The creation of these sites was mainly attributed to the presence of the protonated surface hydroxyls of γ-alumina and not to the neutral hydroxyls. Analysis of the adsorption isotherms suggested that considerable lateral interactions are exerted between the adsorbed Mo x O y z − ions, resulting in the formation of Mo x O y z − … Mo x O y z − … oligomers in the IHP. It is proposed that these polymeric ions are deposited on the surface in the drying step.

On the relationship between the preparation method and the physicochemical and catalytic properties of the CoMo/γ-Al2O3 hydrodesulfurization catalysts

A series of CoMo/gamma-Al(2)O(3) catalysts have been prepared using various methodologies. One of them (EDF) was prepared by depositing the Mo species on the support via the equilibrium deposition filtration (EDF) technique and then the Co species by dry impregnation. Another catalyst (co-EDF) was prepared by depositing the Co and Mo species simultaneously via EDF. A third catalyst (co-WET) was prepared by depositing Mo and Co species simultaneously using the wet impregnation method. The fourth catalyst (WET) was prepared by depositing the Mo species through wet impregnation and then the Co species by dry impregnation. Finally, the fifth catalyst (s-DRY) was prepared by mounting the Mo species through successive dry impregnations and then the Co species by dry impregnation. In all cases the Mo and Co content was identical, giving a Co/(Co+Mo) ratio equal to 0.13. These catalysts were characterized using various physicochemical techniques (BET, NO chemisorption, DRS, LRS, TPR, and XPS), and their catalytic activity for the hydrodesulfurization of thiophene was determined. The trend observed for the HDS activity (namely, EDF>co-EDF>co-WET>s-DRY>WET) is attributed to similar trends observed for both the fraction of well-dispersed octahedral cobalt in the oxidic precursors and the concentration of the edge sulfur vacancies formed on the active phase of the sulfided samples. The EDF and co-EDF catalysts exhibited relatively low hydrogenating activity. The maximum HDS activity, achieved over the EDF catalyst, suggested the most suitable preparative strategy for the preparation of very active and less hydrogen-demanding CoMo/gamma-Al(2)O(3) HDS catalysts.

Fluorinated hydrotreatment catalysts: characterization and hydrodesulphurization activity of fluorine-nickel-molybdenum/γ-alumina catalysts

The oxidic and sulphided state of a series of FNiMo/γ-Al2O3 catalysts containing constant MoO3 and NiO loadings (11.5% and 7.3%, respectively) and various amounts of F−, 0.0–2.0%, introduced after the deposition of the Mo(VI) and Ni(II) were characterized using X-ray photoelectron spectroscopy (XPS), X-ray powder analysis, diffuse reflectance spectroscopy, nitric oxide adsorption, B.E.T. and pore-volume distribution measurements. The results were correlated with the catalytic hydrodesulphurization activity of the specimens, performed in the temperature range 255–325°C using a differential fixed-bed reactor operating under atmospheric pressure. Concerning the oxidic precursor state, the formation of the following species takes place: (i) a supported molybdate phase in which a fraction of the supported Ni(II) has been incorporated in octahedral sites; (ii) α-NiMoO4 and (iii) NiAl2O4. The relative concentrations of these phases are independent of the fluoride content. Low fluoride concentration (≤ 0.8%) has little effect on the texture of the solids. Further increase in the concentration of the modifier results in a marked decrease in the specific surface area which can be partly attributed to the breaking of the inner walls of the relatively thin pores and the creation of larger ones in the range 15–50A. The XPS spectra of the oxide precursors showed that fluorination does not change the structure of the supported phase. No AlF3 was detected. The surface coverage of the active phase was estimated in the sulphided state by the total amount of nitric oxide chemisorbed (per m2 of the catalyst) at 25°C. This decreases slightly up to 0.3 wt% F− and then increases linearly with the fluoride content. The XPS spectra of the sulphided specimens showed that fluorination inhibits the extent of sulphidation and/or reduction of Mo(VI) and the sulphidation of Ni(II) as well. The intensity of both effects increased with the fluoride content. The intrinsic hydrodesulphurization activity was found to increase with the fluoride content. The variation in the activity with the surface coverage of the active phase in the sulphided state showed that the promoting action of F− is mainly due to the fact that these ions increase the surface coverage of the supported Mo and Ni and, therefore, the number of active sites per unit surface area of catalyst. The increase in the surface coverage was attributed to both the increase in the dispersion and the decrease in the specific surface area.

Fluorinated hydrotreatment catalysts Effect of the deposition order of F- ions on F-CoMo/γ-Al2O3 catalysts

Abstract In order to elucidate the influence on the hydrodesulphurization (HDS) activity of the order of deposition of F− ions on CoMo/γ-Al2O3-F catalysts, we prepared, characterized (using BET, NO chemisorption, DRS and XPS) and determined the HDS activity (at atmospheric pressure and various temperatures using the HDS of thiophene as model reaction) of four samples. Specifically, we prepared one undoped CoMo/γ-Al2O3 catalyst (denoted as MC) and three fluorinated catalysts in which the F− ions were deposited before (FMC), after (MCF) and simultaneously with the MoVI and CoII phases ((MCF)). All the specimen which were prepared contained a fixed amount of Mo (11.5 wt.-% MoO3), Co (3.5 wt.-% C03O4) and F (0.8 wt.-% F). It was found that a change in the deposition order provoked a change in the specific surface area, in the dispersity of the supported Mo and Co species and in the intrinsic activity of each HDS active site. The last change was found to be related to the ratio Mo(octahedral)/Mo(tetrahedral). Moreover, it was inferred that fluorination increased the number of active sites per unit active surface. The above explained the changes in the density of the HDS active sites, in the number of these sites and finally on the HDS activity which follows the order MCF>FMC>MC>(MCF) in all temperatures studied. Finally, an attempt was made to convincingly develop pictures related to the preparation which explain the influence of the F doping on the specific surface area and dispersity of the Mo and Co supported species.

Molybdenum-oxo species deposited on titania by adsorption: Mechanism of the adsorption and characterization of the calcined samples

The mechanism of the adsorption of molybdates on the titania surface has been investigated using adsorption equilibrium experiments, potentiometric titrations, and microelectrophoretic mobility measurements. Comparison of adsorption data with the surface charge of titania, regulated by changing the pH of the impregnating solution, demonstrated that the groups responsible for the creation of the adsorption sites are mainly the protonated surface hydroxyls of titania in addition to the neutral sites. Moreover, the results obtained by the combination of potentiometric titrations and microelectrophoretic mobility measurements, and the variation of pH before and after adsorption with the Mo(vI) concentration suggested that the MoO22− ions are adsorbed on the Inner Helmholtz Plane (IHP) of the electrical double layer, which is developed between the surface of the titania particles and the impregnating solution. Finally, from the analysis of the isotherms obtained it was concluded that the adsorbed MoO2− ions are located on energetically equivalent sites of the IHP and that relatively weak lateral interactions are exerted. On the basis of the abovementioned menhanism, an explanation of the dependence of the sorptive capacity of titania on the pH of the impregnating solution is provided. The combined use of NO and CO2 adsorption, as well as XPS, RAMAN spectroscopy, and temperature programmed reduction measurements, showed that at pH 7.3, corresponding to about 2 wt% MoO3, the titania surfaceis completely covered and the active surface reaches its maximum value. At pH values lower than 7.3, a second molybdena layer starts to form until the Mo03 content reaches the 5.4 wt% MoO3, a point at which Mo03 crystallites are formed.

Molybdenum-oxo species deposited on alumina by adsorption: II. Regulation of the Surface MoVI Concentration by Control of the protonated Surface Hydroxyls

Abstract The adsorption of Mo x O y z − species from aqueous solutions on the surface of pure and Na + or Li + -doped γ-aluminas was studied over a pH range between 3.0 and 8.5 and temperatures ranging from 10 to 55°C. The variations in pH with the concentrations of the impregnating solutions observed and analysis of the isotherms demonstrated that the following findings reported for adsorption on pure γ-alumina at T = 25°C and pH 5 are also valid for adsorption on pure and sodium-doped aluminas performed at various pH values and temperatures: (i) The contribution of precipitation to the deposition is negligible. (ii) The adsorption constant for the MoO 4 2− ions is larger than those for the isopolyanions. (iii) The adsorbed Mo x O y z − ions are located on energetically equivalent but distinct sites of the inner Helmholtz plane, created mainly by the protonated surface hydroxyls. (iv) Lateral interactions are exerted between the adsorbed Mo x O y z − ions resulting in the formation of Mo x O y z − … Mo x O z − oligomers. Moreover, it was demonstrated that the regulation of the concentration of the protonated surface hydroxyls makes it possible to deposit by adsorption very large amounts of Mo VI on the γ-alumina surface. Although the sodium doping was found to be the most attractive from the view point of maximization of the extent of adsorption, it may be suggested that it promotes the formation of the catalytically inert sodium molybdate. Therefore the second best method of regulation, namely the change in the impregnating temperature, may prove to be the most convenient in practice.

NiW/γ-Al2O3 catalysts prepared by modified equilibrium deposition filtration (MEDF) and non-dry impregnation (NDI) characterization and catalytic activity evaluation for the production of low sulfur gasoline in a HDS pilot plant

Abstract The aim of this study was to investigate the desulfurization of fluid catalytic cracking (FCC) gasoline using NiW supported on γ-Al 2 O 3 catalysts prepared by conventional non-dry impregnation (NDI) method and a new one, the equilibrium deposition filtration, (EDF). The latter has been modified (MEDF) to produce the required catalyst quantity for its evaluation in a hydrodesulfurization (HDS) pilot plant. The modification of the typical EDF method showed that it is possible to apply the principles of this technique for practical catalyst preparations. In the first part of this work, the catalyst preparation methodology and the catalyst characterization results are presented. In the second part, the catalyst evaluation is described, in relation to a commercial catalyst, concerning their HDS, hydrogenation (HYG) and isomerization (ISO) activities. The main conclusion of this study was that the MEDF method is more promising than the conventional NDI method for preparing NiW hydrotreatment catalysts. The catalyst prepared by MEDF exhibited higher HDS activity than that of the NDI one, comparable to that of a commercial CoMo catalyst and the highest HYG and ISO activities. The enhanced catalytic activity of the NiW–MEDF catalyst was attributed to the high dispersity of the supported phases achieved by the corresponding preparation method.

Effect of temperature on the point of zero charge and surface dissociation constants of aqueous suspensions of γ-Al2O3

The surface acidity constants, the point of zero charge (p.z.c.) and the concentration of the charged surface groups, text-decoration:overlineAlOH+2 and text-decoration:overlineAlO–, of γ-Al2O3 over a wide pH range have been determined at temperatures between 10 and 50 °C by potentiometric titrations. It was found that a precise regulation of the value of the p.z.c. can be achieved in the pH range 4.45–8.95 by varying the temperature of the impregnating solution between 10 and 50 °C. Increasing the temperature in this range caused an increase in the p.z.c. value. These results demonstrate that the increase (decrease) in the temperature of the impregnating suspension is a very simple method for achieving a marked extension of the pH range in which γ-Al2O3 can adsorb negative (positive) species. Moreover, it was observed that an increase (decrease) in the concentration of the text-decoration:overlineAlOH+2(text-decoration:overlineAlO–) groups, which are sites for adsorption of negative (positive) species, is obtained by increasing the temperature of the impregnating suspension. The p.z.c. values obtained experimentally were in excellent agreement with those calculated from the average of the two dissociation constants, Kint1, and Kint2, corresponding to the deprotonation–protonation equilibria of the surface text-decoration:overlineAlOH groups.The influence of temperature of the impregnating suspension on the p.z.c., (text-decoration:overlineAlOH+2) and (text-decoration:overlineAlO–) was interpreted assuming that the surface deprotonation reactions of the text-decoration:overlineAlOH+2 and text-decoration:overlineAlOH groups are exothermic. For the first time values for the enthalpies for these surface reactions were obtained (ΔH°1=–180±10 kJ mol–1, ΔH°2=–230±20 kJ mol–1) by plotting ln Kint1 or 2vs. T–1, thus confirming the abovementioned assumption.